MaxLength only works for TextBox celltypes. Normally, you could use TableCOntrolCurrentCellValidateString to restrict the length but this event is not being raised for comboboxes. http://www.syncfusion.com/support/issues/grid/Default.aspx?ToDo=view&questId=787
Deriving the cell control and trying to handle it within the renderer would be one way to try to do it until a fix for the above defect is published.